1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for producing images of the inside of the body of an examination subject. To that end, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet system in a magnetic resonance apparatus. The magnetic resonance apparatus further has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals, and for picking up the magnetic resonance signals that are generated. Magnetic resonance images are produced on the basis of the received signals.
For generating gradient fields, suitable currents are set in the gradient coils of the gradient coil system. The amplitudes of the required currents thereby amount to up to several 100 A. The current rise and decay rates amount up to several 100 kA/s. Given an existing basic magnetic field on the order of magnitude of 1 T, Lorentz forces act on these temporally varying currents in the gradient coils, these Lorentz forces leading to mechanical vibrations of the gradient coil system. These vibrations are forwarded to the surface of the device via various propagation paths. At the surface, these mechanical vibrations are converted into acoustic vibrations that ultimately lead to inherently unwanted noise.
These vibrations can be analyzed, for example, on the basis of the natural vibrational behavior of the gradient coil system. The natural behavior is determined by the Eigen-frequencies and the natural vibrational modes. The effect of the Lorentz forces on the modes is described in the form of participation factors. These indicate how strongly the Lorentz forces excite a specific mode. Given knowledge of the participation factors and of the Eigen-frequencies, the vibration of the gradient coil system can be defined for every location and for every frequency by a superimposing the vibrations of the individual self-oscillation modes. In particular, a resonant excitation of one of the aforementioned modes due to a pulse sequence of the magnetic resonance apparatus leads to an extremely great amount of noise.
The article by Z. Liu et al, xe2x80x9cLocally Resonant Sonic Materialsxe2x80x9d, Science, Vol. 289, Sep. 8, 2000, pages 1734 through 1736, describes a material with which sound waves that exhibit a wavelength, or a wavelength range around a wavelength can be highly efficiently reduced in amplitude given layer thicknesses of the material that are one through two orders of magnitude smaller than the wavelength. Due to resonant effects, the material thereby an effectively negative elasticity constant, at least for the wavelength range. The material thus can be tuned to a prescribable wavelength or frequency range that is to be reduced. It is described as an example in this article that an amplitude attenuation of the oscillation by more than three powers of ten can be achieved for a 400 Hz oscillationxe2x80x94which corresponds to a wavelength of approximately 83 cm in airxe2x80x94with an approximately 2 cm thick layer of the material.
A great number of passive and active noise-reduction measures are known for magnetic resonance apparatuses. Included, for example, among known, passive noise reduction measures are the application of sound-damping foam materials to cladding parts facing toward the gradient coil system and/or the use of flexible layers at and/or in the gradient coil system. Such measures are disclosed, for example, U.S. Pat. No. 4,954,781.
An object of the present invention is to provide an improved magnetic resonance apparatus that, in particular, exhibits low noise emission values.
This object is inventively achieved in a magnetic resonance apparatus having a sound-proofing structure that contains a material having an effectively negative elasticity constant.
Due to the employment of a soundproofing structure containing a material having an effectively negative elasticity constant, noise emission by the magnetic resonance apparatus can be highly effectively reduced at least for a prescribable frequency range with layer thicknesses of the soundproofing structure that are one through two orders of magnitude smaller than the appertaining wavelengths of the prescribable frequency range. As a result, a high noise-reducing effect can be achieved with a soundproofing structure having a small additional volume and a small additional mass. Further details about the material are described, for example, in the initially cited article by Z. Liu et al.